Enantio- and diastereoselective catalytic carboalumination of 1-alkenes and alpha,omega-dienes with cationic zirconocenes: Scope and mechanism

Bis(pentamethylcyclopentadienyl) dimethylzirconium (Cp-2*ZrMe2) and bis(1-( -)-neomenthylindenyl) dimethylzirconium ((NMIn)(2)ZrMe2) upon activation by tris(pentafluorophenyl)borane are efficient catalysts for the carboalumina tion of a variety of l-alkenes and nonconjugated dienes. The organoalane pr oducts can be oxidized to alcohol products, which are isolated in 50-90% yi eld. (NMIn)(2)ZrMe2 carboaluminates l-alkenes and dienes with moderate (55- 65%) enantioselectivities. In contrast, ethylenebis(tetrahydroindenyl) dime thylzirconium ((EBTHI)ZrMe2) and related zirconocenes give low activities a nd selectivities in the carboalumination reaction. Study of the interaction of zirconocene methyl cations with trimethylaluminum by H-1 NMR shows that [(EBTHI)ZrMe][MeB(C6F5)(3)] forms a stable bis(mu-methyl) aluminum adduct. Zirconocene methyl cations derived from Cp-2*ZrMe2 and (NMIn)(2)ZrMe2 reac t slowly with large excesses of trimethylaluminum to form weakly bound trim ethylaluminum adducts. The difference in trimethylaluminum binding to the c ationic zirconocene centers is consistent with the significantly lower acti vity observed when (EBTHI)ZrMe2 is used as the precatalyst.